David Brin has a brilliant series starting with Sundiver, The Uplift War & about 3 more. Near future with Man heading out into a hostile crowded galaxy with uplifted chimps & dolphins. Great plots, fascinating characters, imaginative, self-consistent aliens & galaxy-wide conspiracies.
Larry Niven for a tale-teller's series in the Known Universe line-up. Larry takes hard science, wraps it in wonder & makes it everyday. Want to know what happens when you closely orbit a neutron star? How about aliens that you just want to be with because they're so interesting. (& dangerous in the case of Kzinti, which BTW, Star Trek wanted to use & when Niven said no, we got Klingons.
If you haven't read them, the Ringworld series, Legacy of Heorot & Beowulfs Children, The Mote in God's Eye & The Moat Aournd Murcheson's Eye are all highly recommended.
Greg Bear's Eon & Eternity
for sheer exuberant expansion of the universe you thought you'd come to know. For a Stone that will blow your mind - Eon! Then blow it again with Eternity.
Forge of God & Anvil of Stars - aliens arrive, but they contradict each other & there's strange things happening around the world. You won't believe where this pair of books takes you.